Photonic crystal materials with a band gap property responsive to external stimuli have important applications in bio- and chemical sensors, color paints and inks, reflective display units, optical filters and switches, and many other active optical components. Colloidal crystals, which can be produced conveniently by self-assembling uniform colloidal particles, have been particularly useful for making responsive photonic materials because active components can be incorporated into the crystalline lattice during or after the assembly process. The majority of research in the field therefore has been focused on tuning the photonic properties of colloidal systems through changes in the refractive indices, lattice constants, or spatial symmetry of the colloidal arrays upon the application of external stimuli such as chemical change, temperature variation, mechanical forces, electrical or magnetic fields, or light. However, wide use of these systems in practical applications is usually hampered by slow and complicated fabrication processes, limited tunability, slow response to the external stimuli, and difficulty of device integration.
Because the photonic band gap is highly dependent on the angle between the incident light and lattice planes, an alternative route to tunable photonic materials is to use external stimuli to change the orientation of a photonic crystal. For easy fabrication, actuation, and broader applications, it is highly desirable that the photonic crystals can be divided into many smaller parts whose orientation can be controlled individually or collectively as needed by using external stimuli. Photonic crystal microspheres, or “opal balls”, have been previously demonstrated by Velev et al. in a number of pioneering works by using monodisperse silica or polystyrene beads as the building blocks (Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 2240-2243; Rastogi, Melle, S.; Calderon, O. G.; Garcia, A, A.; Marquez, M.; Velev, O. D. Adv. Mater. 2008, 20, 4263-4268). The brilliant colors associated with these three dimensional periodic structures, however, cannot be tuned due to lack of control over the orientation of the microspheres. Xia et al. have introduced magnetic components into a photonic microcrystal so that its diffraction can be changed by rotating the sample using external magnetic fields (Gates, B.; Xia, Y Adv. Mater, 2001, 13, 1605-1608). However, it has not been demonstrated that one can synthesize multiple copies of such microphotonic crystals, align them synchronically, and collectively output uniform color signals.